Mid-infrared lasers are important for a growing number of applications, such as spectroscopy, remote chemical sensing, laser surgery, and infrared countermeasures. Typically, laser output at mid-infrared wavelengths is produced by using a nonlinear optical (NLO) crystal to shift the output wavelength of a solid state laser, such as a Nd:YAG (1.06-μm) laser or an erbium-doped fiber laser (1.55-μm), to a wavelength in the 2-12 μm range. Oxide-based crystals such as potassium titanyl phosphate, KTiOPO4 (KTP), and periodically-poled lithium niobate, LiNbO3 (PPLN) work well at the short end of this spectral range, but the output power from these materials falls off dramatically beyond 4 microns.
The I-III-VI2 chalcopyrite crystals AgGaS2 and AgGaSe2 can be pumped at 1.06 μm and 1.55 μm respectively so as to generate output much deeper into the infrared (up to 12 μm), but they are plagued by low damage thresholds and extremely poor thermal properties that preclude their use for high average power applications. Crystals of ZnGeP2, a II-IV-V2 chalcopyrite, have significantly higher nonlinear coefficients and much better thermo-mechanical properties for high power operation, but they must be pumped by less common sources such as Tm- or Tm,Ho-lasers operating at wavelengths greater than 1.9 μm. Furthermore, the output power and efficiency of 2-micron-laser-pumped ZnGeP2 optical parametric oscillators (OPOs) is limited by absorption losses at the pump wavelength.
II-IV-V2 chalcopyrites like ZnGeP2 are derived from III-V compound semiconductors (like GaP) by ordered substitution of group II (Zn) and group IV (Ge) atoms on the group III (Ga) site in the same way that I-III-VI2 chalcopyrites like AgGaS2 and AgGaSe2 are derived from II-VI compound semiconductors (like ZnSe). While the I-III-VI2 compounds offer low absorption losses, they are usually plagued by scattering losses due to precipitates which form on cooling as a result of off-stoichiometry congruent melting and retrograde solubility of the III2VI3-rich phase. They tend to have larger band gaps than their II-IV-V2 counterparts, but are inferior in terms of hardness, thermal conductivity, anisotropic thermal expansion, and thermo-optic coefficients, all of which makes them prone to thermal lensing and laser-induced damage in both the surface and the bulk. They also have lower nonlinear coefficients than the II-IV-V2 chalcopyrites.
The II-IV-V2 compounds, therefore, offer superior properties in almost every respect. They are generally free of scattering centers, and absorption losses (at least in the phosphides) can be very low in the center of their transparency range. While they can be plagued by defect-related absorption losses near the band edge when grown from the melt, these losses can often be eliminated by post-growth processing.
Of course, not all II-IV-V2 compounds are suitable for nonlinear optical applications. To be useful for nonlinear optical frequency conversion, a crystal must have sufficient birefringence for phase-matching. Because the speed of light (as determined by the refractive index, n) inside a crystal varies as a function of wavelength (a phenomenon known as “dispersion”), the input and output waves will normally remain in phase over a very short distance (˜10-100 μm in the infrared), termed the “coherence length,” beyond which frequency conversion ceases. A birefringent crystal exhibits two refractive indices: one for light polarized perpendicular to the optic axis (ordinary- or o-polarized) and another for light polarized parallel to the optic axis (extraordinary- or e-polarized). If the difference between the two refractive indices (i.e., the birefringence, ne−no) exceeds the difference in refractive index at the two wavelengths (i.e., the dispersion), then a direction inside the crystal can be chosen such that input and output waves of opposite polarization can experience the same refractive index. Under these conditions, the two waves remain in phase (i.e. are “phase-matched”), allowing the frequency conversion process to build to reasonable efficiencies over a long interaction length.
In addition to phase matching, a nonlinear optical crystal must be highly transparent at the input and output wavelengths of interest, and it must have a band edge that is near or below one-half of the shortest wavelength involved in the frequency conversion process, so as to avoid two-photon absorption. For down-conversion processes such as optical parametric oscillation (OPO), optical parametric amplification (OPA), and difference frequency generation (DFG), the input wave represents the shortest wavelength, whereas for up-conversion processes such as second harmonic generation (SHG) or sum frequency generation (SFG) the shortest wavelength(s) is/are at the output. Pumping with a 1064 nm Nd:YAG solid state laser, for example, requires a band edge near or below 532 nm.
Finally, even if a candidate compound has adequate nonlinearity, transparency, and phase-matching properties, it will not be possible to use the compound as the basis of an optical device unless a method is found for growing crystals of sufficient size and optical quality so that devices with the required crystallographic orientation can be fabricated. Oriented crystals with dimensions of at least 3 mm×3 mm×10 mm are typically required for practical devices, and the crystals must be free of cracks, twins, voids, inclusions, grain-boundaries, and other macroscopic defects, at least within the aperture and propagating path of the interacting waves.
Although at the time the present invention was made, CdSiP2 was known to be a II-IV-V2 chalcopyrite compound with at least some properties consistent with use in NLO devices, CdSiP2 was not generally considered by those skilled in the art to be a likely candidate for practical NLO devices. The crystal structure of CdSiP2 had been reported by S. C. Abrahams and J. L. Bernstein, J. Chem. Phys. Vol. 55, p. 796 (1971), incorporated herein by reference for all purposes. Specifically, Abrahams and Bernstein had reported that CdSiP2 belongs to the tetragonal space group 42m, that its lattice parameters are respectively 5.68 A and 10.431 A for a and c, and Z=4 in each unit cell, and that the mass density of CdSiP2 is 4.70 g/cm3.
The birefringence of tiny 2×2×0.2 mm3 CdSiP2 crystals grown from a molten tin flux had been measured in the prior art and reported in N. Itoh, T. Fujinaga, and T. Nakau, “Birefringence in CdSiP2,” Jap. J. Appl. Phys. 17, 951-2, (1978) (incorporated herein by reference for all purposes) to be −0.045 at 840 nm, but no measurements were made in the infrared.
Several other optically relevant properties of CdSiP2 were either uncertain or unknown at the time the present invention was made. For example, values reported in the prior art for the band gap of CdSiP2 had ranged from 2.2 eV (563 nm) to 2.45 eV (506 nm), causing it to be unclear if CdSiP2 would exhibit two-photon absorption if pumped by a pulsed 1064 nm laser. The long-wavelength transparency limit of CdSiP2 had not been measured in the prior art. And while tiny samples (up to 5×2×1 mm3) had been grown in the prior art by halogen-assisted vapor transport, as reported for example in E. Buehler and J. H. Wernick, Journal of Crystal Growth 8, 324 (1971), incorporated herein by reference for all purposes, the CdSiP2 crystals produced thereby were far too small to be used for nonlinear optical devices. In fact, these prior art crystals were too small to enable measurement of the most relevant nonlinear optical properties, including the nonlinear coefficient, the transparency range, and the phase-matching characteristics. Furthermore, the methods used to grow these prior art crystals were not scalable to produce crystals having the required size and quality.
At the time the present invention was made, it therefore appeared doubtful to those of average skill in the art that CdSiP2 crystals of optical device size and quality could be grown. Buehler and Wernick had reported an approximate melting point of 1120° C. for CdSiP2 and a vapor pressure of 19.4 atm at 1100° C. (which extrapolates to 20.4 atm at 1120° C. and 23.9 atm at 1175° C.). The present inventors had even attempted to grow CdSiP2 directly from a stoichiometric melt without success, finding that the quartz ampoules devitrified and reacted with CdSiP2, and that the solidified melts were porous and polycrystalline. An equilibrium phase diagram for CdSiP2 system had not been determined in the prior art, and it was not known if the compound would melt congruently, which is a necessary condition for melt growth.
A method was known in the prior art (see U.S. Pat. No. 5,611,856, incorporated herein in its entirety for all purposes) for producing single crystals of group II-IV-V2 compounds of sufficient size and optical quality for use in NLO devices by pre-synthesizing the compound material from its constituents and separately melting and re-crystallizing by directional solidification in sealed quartz ampoules using a two-zone horizontal transparent furnace. Crystals with melting points as high as 1027° C. and vapor pressures as high as 7 atmospheres had been grown by this method. However, it did not appear at the time of the present invention that this method would tolerate the significantly higher temperatures (˜1175° C. in the hot zone) and pressures (˜24 atm) required to produce a CdSiP2 crystal. In addition, it was not known whether the compound itself would melt congruently, and whether the quartz ampoules could be used without devitrifying or reacting with the compound. Finally, it was not apparent that a crystal of CdSiP2 would be transparent and phase-matchable for mid-infrared frequency conversion, even if a crystal of sufficient size and quality could somehow be grown.
For all of these reasons, at the time the present invention was made CdSiP2 was not generally considered by those skilled in the art to be a likely candidate crystal for practical NLO device applications.
A need exists, therefore, for CdSiP2 single crystals of sufficient size and optical quality to be suitable for use in NLO devices, and for a method of producing such crystals. A need also exists for NLO devices that use such CdSiP2 crystals to frequency-shift the output of 1.06 μm and 1.55 μm lasers to mid-infrared wavelengths ranging from 2-10 μm, and for frequency-shifting 2-μm lasers at high efficiencies and output powers into the 3-10 μm range. Furthermore, a need exists for a method of using such crystals of frequency-shifting the output of 1.06 μm, 1.55 μm, and 2-μm lasers to mid-infrared wavelengths ranging from 2-10 μm.